100 Mbps baseband Fast Ethernet specification using UTP wiring. Like the 10BaseT technology on which it is based, 100BaseT sends link pulses over the network segment when no traffic is present. However, these link pulses contain more information than those used in 10BaseT.

Add/Drop Multiplexer. A device which inserts lower rate channel traffic [add] or removes the traffic [drop] from a higher rate aggregate channel in a synchronous transmission network (SDH or SONET). Typically adds or drops traffic at STM-1 or higher from a STM-64 aggregate signal.

ADSL

Asymmetric Digital Subscriber Loop. A family of technologies used to deliver high-rate digital data over the existing copper local loop. One-way rates of up to 6 Mbps downstream (from the central office to the subscriber) and up to 640 kbps upstream have been achieved, although the technology is still evolving and higher data rates are expected in the future.

ADSL is expected to grow to become the main connection to the Net for the home and small business user due to the relatively low cost of the technology.

Asynchronous Transfer Mode. ATM is a connection-oriented packet switching technique where all cells follow the same path through the network. Cells are made up of 48 bytes of data plus a 5 byte header (53 bytes total). See www.atmforum.com for more details.

Availability

The percentage of time for which a circuit or system is
available for use. Carrier class is typically 99.999% - about five minutes downtime per year.

Device that connects two or more networks together and forwards packets between them. The networks may use dissimilar protocols. A bridge operates at the data link layer (layer two) as opposed to a router which operates at layer three.

A statement of system or network availability. Carrier class is typically 99.999% - about five minutes of downtime per year.

CIR

Committed Information Rate. A specified amount of guaranteed bandwidth (measured in bits per second), usually on a Frame Relay service. Typically, when purchasing a Frame Relay service, a company can specify the CIR level they wish. The Frame Relay network vendor guarantees that frames not exceeding this level will be delivered. It's possible that additional traffic may also be delivered, but it's not guaranteed. Some Frame Relay vendors offer inexpensive services with a CIR equal to zero. This essentially means that the network will deliver as many frames as it can, but it doesn't guarantee any bandwidth level.

CPE

Customer Premises Equipment. This is the equipment at the customer site which terminates the connection into the network and connects to the LAN for private networks, Internet connectivity and voice services.

A fiber-optic strand with no optical transmission equipment. Customers add their own equipment and build their own network, retaining complete control over all aspects of it.

Datagram

The basic unit of information passed across the Internet. It is a self-contained packet containing source and destination addresses as well as data. See also frame and packet.

Data Link
Layer

Second layer in the OSI seven layer model. Layer Transmits packets from node to node.

dB

deciBel. A logarithmic unit which defines the ratio between two powers P1 and P2. Ratio in dB = 10 log10 (P1/P2). The original unit was the Bel, named after Alexander Graham Bell, but is inconveniently large.

Used in power budgets to describe the optical loss over a particular link and therefore the optical power needed over the link in order to get a signal to the other end.

Used in fiber specifications to describe the loss per meter in the fibre, where the lower the loss the better.

DBS

Direct Broadcast Satellite - where the end-user receives the broadcast signal direct from the satellite. As opposed to a satellite feed to a cable TV head end, for subsequent distribution to end-users via the cable network.

DCN

Data Communication Network. Used to convey Network Management commands and reports around a communications network infrastructure.

DHCP

Dynamic Host Configuration Protocol. A method of assigning an IP address dynamically to a device each time it connects to a network. DHCP simplifies network administration and means that normal PC's on a network do not need a fixed IP address.

Dispersion

Normally refers to chromatic Dispersion. Chromatic Dispersion is the variation of propagation velocity - speed of travel of the light - in an optical fiber with frequency - and with wavelength - causing the pulses of light to be degraded and merge into each other as the light travels down the fibre. See also PMD. Different types of fiber have different Dispersion characteristics. Management of Dispersion is key to network design at the fiber level.

DMZ

Demilitarized Zone. Part of the network to which access is controlled by a Firewall.

DNS

Domain Name System. A distributed database which provides the mapping/translation between the domain name and the individual IP address allocated to that host.

Domain Name

The domain name is the unique name that identifies an organization on the Internet. The Fully Qualified Domain Name (FQDN) includes the hostname and is easier to use than the numerical IP addresses which are used to route traffic to the correct destination. This means that you only need to remember the name of the web site rather than its IP address.

A given server may have or support more than one domain name, but a given FQDN points to only one host.

com

Commercial

edu

Educational

gov

Government (US unless specified)

int

International

mil

Military (US unless specified)

net

Network

org

Non-profit organization

Subsequently the system was extended to allow an optional final two-letter country group. Examples are given below.

Dense Wavelength Division Multiplexing. Multiplexing of signals by transmitting them on typically eight or more different wavelengths on the same fibre. The ’dense’ prefix usually signifies systems where the spacing between wavelengths is smaller than the first generation WDM systems.

Erbium Doped fiber Amplifier. Optical fibres doped with the rare earth element erbium, which can amplify light in the wavelength region from 1530nm to 1625nm when pumped by an external light source. EDFAs allow an optical pulse to travel long distances before electrical regeneration is required.

Ethernet

A LAN access method defined in IEEE 802.3. It is a shared medium approach originally developed at Xerox Palo Alto Research Center using ideas from the University of Hawaii Aloha packet radio network. Available at 10 Mbps (10BaseT), 100 Mbps (100BaseT) and now 1 Gbps (Gigabit Ethernet).

Forward Error Correction. Technique for detecting and correcting errors (from imperfect transmission) by adding a small number of extra bits. FEC allows optical transmission over longer distances by correcting errors that can happen as the signal-to-noise ratio decreases with distance. See also Raman Amplification.

Fibre-opticCable

Network cabling that employs one or more Optical Fibres.

Firewall

Device placed between an organization’s private network and the Internet to authenticate incoming users. A Firewall is normally a specially configured computer which is set up to only allow specific incoming traffic and users onto the network.

Frame

1. A block of data in a specified format.
2. A rack housing telecommunications equipment.

Frame Relay

High-speed packet switching technique used to interconnect LANs. Capable of any payload up to 4096 bytes per packet. Defined in ITU-T I.122. Typically used to build VPN's, particularly where guarantees of bandwidth are required.

FWM

Four Wave Mixing. Describes the generation of unwanted sidebands when two or more high-power optical signals exist in the same non-ideal medium.

FWM is one of the elements that need to be taken into account when designing optical networks, and must be minimized if interference is to be avoided.

FTP

File Transfer Protocol. Operates at layers five, six and seven of the OSI model and allows log-on to a remote host, directory listing and file transfer.

HyperText Markup Language. Simple hypertext document formatting language that uses tags to indicate how a given part of a document should be interpreted by a viewing application, such as a Web browser.

HTTP

HyperText Transfer Protocol. Part of the TCP/IP suite. The underlying protocol of WORLd Wide Web pages, used to communicate between the user’s browser and the Web server.

Internet Architecture Board. Board of internet work researchers who discuss issues pertinent to Internet architecture. Responsible for appointing a variety of Internet-related groups.

IETF

Internet Engineering Task Force. Task force consisting of over 80 working groups responsible for developing Internet standards (produced as RFC's). The IETF operates under the auspices of ISOC.

ILA

In-Line Amplifier.

Internet

The interconnection of computers across the world that evolved from the ARPAnet (Advanced Research Projects Agency).

The Internet is made up of many networks each run by a different company and interconnected at peering points. The common use of IP and Internet standards allows users connected to one network to communicate with users on another network.

IP

Internet Protocol. Defines the unit of information passed between systems providing a basic packet delivery service within the TCP/IP. IP is a standard that describes how packets of data are transported across the Internet and recognized as incoming messages.

IPSec provides security for transmission of sensitive information over unprotected networks such as the Internet. IPSec acts at the network layer, protecting and authenticating IP packets between participating IPSec devices.

Internet Society. International non-profit organization founded in 1992 to coordinate the evolution and use of the Internet. In addition, ISOC delegates authority to other groups related to the Internet, such as the IAB.

Local Area Network. Used within a building to link computers and other devices, such as printers. Typically uses Ethernet. Click here for diagram.

Latency

Time taken to deliver a packet from the source to the receiver. Includes propagation delay (the time taken for the electrical or optical signals to travel the distance between the two points) and processing delay. Due to the distance to a satellite and back (over 34,000km each way), the latency when communicating via a satellite connection is at least 270 milliseconds, making interactive services difficult, compared to a delay of about 10 milliseconds across Europe via fibre.

Local Loop

Originally, the pair of wires (loop) between the subscriber (to a telephone system) and the local telephone exchange (switch or office). Now used as a generic term to describe the connection between the last switch/routing point and the subscriber, no matter what technology is used to deliver the service, nor what service (voice, data etc.) is delivered.

Mode Field Diameter of an optical fibre. The diameter at which the electric and magnetic field strengths are reduced to 1/e of their maximum values (for a Gaussian distribution in a single mode fibre, and where e is the base of natural logarithms, 2.71828...). This is the practical replacement for core diameter in single-mode fibre.

MIB

Management Information Base. Database of Network Management information that is used and maintained by a Network Management protocol such as SNMP or CMIP. The value of a MIB object can be changed or retrieved using SNMP or CMIP commands, usually through a GUI Network Management system. MIB objects are organized in a tree structure that includes public (standard) and private (proprietary) branches.

MultiProtocol Label Switching. A short fixed-length label is generated that acts as a shorthand representation of an IP packet's header. Subsequent routing decisions (made by Label Switched routers) are made based on the MPLS label and not the original IP address. This new technology allows core network routers to operate at higher speeds without needing to examine each packet in detail, and allows more complex services to be developed, allowing discrimination on a QoS basis.

MS

Multiplex Section

MS-SPRing

Multiplex Section Shared Protection Ring

MTBF

Mean Time Between Failures. A key metric for the quality of equipment and a determining factor in the overall SLA that can be achieved.

MTTR

Mean Time To Repair. The usual time taken to fix a problem that occurs on the network. Targets are normally set within an SLA and depend on the priority of the fault.

This is the point where the optical fiber within the backbone network terminates and the customer's equipment of fiber connects, providing the demarcation point between i-21 and the customer.

Optical Fiber

A method of guiding light over long distances with very little reduction on strength (attenuation or loss). A central core of high-refractive index material - usually very pure glass - is covered with a cladding of lower refractive index material. Modern fibres have loss in the order of 0.25 dB/km, so 1 km of fiber has less loss than a pair of ordinary spectacles or reading glasses.

Optronics

From Optoelectronics. The combination of optics and electronics.

ORL

Optical Return Loss. Ratio of power reflected (from a connector or other discontinuity) to incident power. Usually expressed in dB.

A block of data. The terms packet, frame, and datagram are often used interchangeably.

Peering

Internet interconnection as equals (peers) and thus no billing between the parties. This is the normal method of interconnection between the sub-networks which make up the Internet. Contrasts with supplier/ customer interconnection, e.g. ISP and end customer.

First layer in the OSI seven layer model. The electrical or optical signals physically transported across the network.

Plesiochronous

Almost synchronous because bits are stuffed into the frames as padding and the call’s location varies slightly - jitters - from frame to frame.

PMD

Polarisation Mode Dispersion. Dispersion caused by different material properties for different planes of polarisation (direction of the electric field) in an optical fibre.

PoP

Point of Presence (or Access Node). A site where customers can connect into the backbone network.

POTS

Plain Old Telephone Service

Power Budget

The optical budget in dB over a specific network link. Usually quoted as an ’end-of-life’ figure which takes into account some margin for repairs and expected very small deterioration in the quality of the fiber over time.

A technique for amplifying optical signals in which high-power laser light is sent in the direction opposite that traveled by the data signals, transforming part of the transmission fiber into an amplifier of the signals passing through it. Raman Amplification is named after the scientist who discovered the phenomenon in the scattering of light, called the Raman Effect, in 1928. Typically used to extend the distance that optical signals can be transmitted. See also FEC.

Request For Comments. Document series used as the primary means for communicating information about the Internet. Some RFC's are designated by the IAB as Internet standards. Most RFC's document protocol specifications such as Telnet and FTP but some are humorous or historical. RFC's are available online from www.rfc-editor.org.

RIPE

Réseaux IP Européens. Group formed to coordinate IP based networks in Europe. This is the organization within Europe that allocates IP addresses to ISP's who in turn allocate them to customers.

Router

Device connecting two or more networks together which forwards packets between them. Routers read the network address and use routing tables to find the best route between the networks. The routing tables can be created automatically by the system. Routers can also implement load balancing and generate statistics. A router operates at the network layer (layer three) as opposed to a bridge which operates at layer two.

Type of optical connector. Type SC, Super Physical Contact. The type SC connector was originally developed by NTT of Japan. The suffixes /PC, /SPC, /APC are terms which describe connector end-faces and also relate to the ORL designation.

PC means Physical Contact, a description of the contacting spherical end-face. PC has come to mean an ORL greater than 35db. SPC means Super PC, which means a PC connector with ORL >45db.

APC means Angled PC (the end face is polished at an angle, usually 8°) which improves ORL to >65db.

Le Système International (d’Unités). The International system (metric) of units of measure.

SMDS

Switched Multimegabit Data Service

SMTP

Simple Mail Transfer Protocol. Part of the TCP/IP suite. This is the protocol used for transporting email over the Internet, between email servers (which hold and store email) and clients (which allow users to read the email).

SNMP

Simple Network Management Protocol. An application layer protocol that facilitates the exchange of management information between network devices. Part of the TCP/IP suite.

Virtual Private Network. A network provided to the customer which is invisible to any other users of the backbone network.

This provides security to the customer (if the network cannot be seen it cannot be interfered with) and allows Quality of Service specific to the customer to be provided. VPN's allow the customer’s view of the network to be greatly simplified and tailored to specific requirements.

VoIP

Voice over IP. The ability to carry Packetised voice over an IP-based Internet with POTS-like functionality, reliability and voice quality.

Wide Area Network. As distinct from Local Area Network. A WAN connects multiple LANs together. Typically an ISP or service provider provides the WAN into which a company will connect their LANs from each site.

WAP

Wireless Application Protocol. Delivers information and services to www.wapforum.org/. This is essentially a minimal form of the Web for mobile phones.

WDM

Wavelength Division Multiplexing. Generic term for the technique of simultaneously transmitting more than one wavelength of light down an optical fibre. Also see DWDM